However, what is the reason for constant max apertures? It's not as if these lenses are controlled by mechanical aperture rings with numerical markings, so ease of operation does not apply. With modern feedback controlled flashguns, the flash control motivation is also removed or significantly weakened. The only possible remaining reason for this would be marketing.

Marketing has got to such a degree that customers, when stating technical desires, are including pure marketing items in their wish lists!

Why would it be a marketing ploy to design a lens that never changes aperture? I am actually not sure I understand how you can't see a constant aperture as desirable.

If you had the choice between, e.g., a 100–200mm f/4 or a 100–200mm f/2.8–4 of approximately the same size and optical performance, which would you prefer?

It's not quite a free lunch — the faster aperture at the wide end might require additional optical correction — but lens size for telephotos is primarily driven by maximum focal length and aperture at that focal length, so it's pretty close.

For WA zooms, things are more complicated, but I think the case for a 12–60mm f/2.8–4 over f/4-constant is very strong. If you don't like f/2.8 on the wide end, you can always stop down!